PURPOSE: To improve operability and to reduce the load on a computer, by performing data backup of a main storage device independently of the computer and the operator.

CONSTITUTION: At the time of normal operation, a disc controlling command is inputted from a computer 11 to a command converter 14 and a controlling device 15 controls a magnetic disc device 12 to store and read out data. When a main power supply device 16 is disconnected at the completion of ordinary service, a main power supply disconnection detecting signal Sc is outputted from a command converter 14, a data transfer command is transmitted to the controlling device 15 and then data in the magnetic disc device 12 are transferred to a magnetic tape device 13. After completing the data transfer, an auxiliary power supply device 17 is disconnected to complete the whole operation.